In various medical contexts it may be desirable to acquire position and/or orientation information for a medical instrument, implant or device that is navigated or positioned (externally or internally) relative to a patient. For example, in surgical and/or interventional contexts, it may be useful to acquire position and/or orientation information for a medical device, or a portion of a medical device, when the device or relevant portion is otherwise out of view, such as within a patient's body. Likewise, in certain procedures where an imaging technique is used to observe all or part of the position and orientation information, it may be useful to have position and orientation information derived from the tracked device itself that can be related to the image data also being acquired.
Because some sensors operate based on electromagnetism, one issue that can arise with navigation sensors suitable for acquiring position and orientation information in this manner is the position and orientation errors that may be caused by the presence of metallic objects in the field of navigation. Unfortunately, these errors can result in incorrect positioning of a medical device during surgical or interventional procedures. Because of the relative ubiquity of metallic devices in such contexts, distortion is often unavoidable. Accordingly, it may be desirable to use systems that operate in an error free manner, even in environments that contain metallic objects. While, it may be desirable to use a navigation system that is able to compensate for distortions caused by metallic objects, in practice this may be difficult because the parameters of the metallic objects causing the distortion may be unpredictable.